1. Field of the Invention
The present invention relates to a transmission power control device in a radio communication transmitting/receiving station particularly for mobile radio systems. The station is notably a mobile station of radio-telephone type on board a vehicle, designed to exchange information, such as voice and/or data messages, with a fixed base station in a telecommunication network operating along a known access process.
2. Description of the Prior Art
In r.f. communication systems with mobile stations, information exchanged between the fixed and mobile stations is transmitted bidirectionally through two radioelectric channels subject to fluctuations, which are to a great extent random. The fluctuations generally result from the combination of three factors:
attenuation due to the variable distance between the mobile and fixed stations, affecting the mean value of the power received in each station for a given distance;
relatively slow variations around the mean power value, due to masks or shadow regions on the radio electrical paths; and
a rapid fluctuation in the signal received due to multiple paths, known as Rayleigh phenomenon, and practically entirely unforeseeable.
Generally, speaking, to ensure maximum power at the fixed station receiver, it is necessary to increase the rated power transmitted by the mobile station by a safety margin to cover the above fluctuations. To save as much of the energy available for transmitting radioelectric signals from mobile station, it is possible to control the power transmitted from the mobile stations to obtain a mean receiving level that is as constant as possible at the fixed station. The control device of the invention remedies the two first attenuation factors, and does not take into account the practically unforeseeable rapid Rayleigh fluctuations.
At the present time, to deal with fluctuations in the received signal, the received signal is generally corrected by automatic selective frequency equalization to remedy effects due to multipath fading, with automatic gain control (AGC) in a receiving amplifier to remedy the slow variations.
In certain radio communication systems each mobile station has several transmission power levels, and commutation is applied after the fixed station has detected saturation of the receive power level, or on the contrary a too low power level, so as to ensure correct receiving. The fixed station transmits a power commutation signal to the mobile station through a radioelectric signally channel. The power commutations are thus totally intermittent in the mobile station.
The aforesaid radio communication systems have a major drawback because, each mobile station communicating with the fixed station is in a different variable position. Thereby the propagation conditions differ from one location to the other, and the level of power received at the fixed station varies considerably according to the location of the mobile station. This makes receiving means in the fixed station more complex.
Moreover, as a safety measure, the mobile station transmits a power which is often well above that required to ensure correct receiving in the fixed station. This implies:
firstly an unnecessarily high energy consumption, which can be critical for a mobile station whose power resources are by their very nature highly limited;
secondly the superfluous power transmitted by the mobile station increases the noise level in the link to the fixed station; this may jam communications to other mobile stations in a service cell containing the fixed station.